Epitaxial LaNiO3(LNO) thin films on LaAlO3(LAO), SrTiO3(STO), and YSZ are grown by pulsed laser deposition method at 350 mTorr oxygen partial pressure and 700 °C substrate temperature. As-deposited LNO films are metallic down to 10 K. c-axis oriented YBa2Cu3O7 (YBCO) films were grown on LNO/LAO as well as LNO/STO surfaces without affecting superconducting transition temperature of YBCO. Textured LNO thin films were grown on c-axis oriented YBCO/STO and YBCO/YSZ . Transport measurements of these bilayer films showed that LNO is a good metallic contact material for YBCO.

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Most materials in their nanocrystalline state exhibit unusual properties. The tremendous enhancement of mechanical, physical, and chemical properties caused by the fine admixture of order (inside the crystallites) and disorder (in the grain boundary regions) is being exploited in a number of frontier technologies. A concern in the use of nanocrystalline materials is their stability during high temperature exposure, either while processing or in service. There is no experimental data in the literature on the Gibbs energy of formation of any nanocrystalline material and no standard technique is available for measurement, especially at elevated temperatures. Quantitative information on the thermodynamic driving force for degradation to the microcystalline state and mechanistic understanding of the degradation processes would be useful in the design and processing of nanocrystalline materials for new applications. Reliable information on surface and interfacial properties of ceramic materials, which provide important guidelines for understanding nanocrystalline materials, are not available in the literature, even for model systems. Magnesium aluminate, which is used as an adsorbent for removing surfactants from aqueous solutions, as a support material for transition metal and lanthanide phosphors, and as a catalyst, was selected as a model compound in this study.Reported in this communication is a novel application of the solid state electrochemical technique for probing the thermodynamic state of nanocrystalline MgAl 2 O 4 at high temperatures. The method has been used recently for the measurement of the Gibbs energy of microcrystalline MgAl 2 O 4 .[1] Although the principle of this method for thermodynamic measurements was outlined in the classic papers of Kiukkola and Wagner, [2,3] the technique has not been applied to nanocrystalline materials. The reversible electromotive force (emf) of the cell was measured as a function of temperature in the range 900 to 1250 K. The cell is written such that the right-hand electrode is positive. Single crystal CaF 2 was used as the solid electrolyte and the cell was operated under dry oxygen. The reference electrode consisted of an intimate equimolar mixture of MgO and MgF 2 powders, compacted at a pressure of 250 MPa. Similarly, the working electrode was a compacted equimolar mixture of MgAl 2 O 4 , a-Al 2 O 3 , and MgF 2 . The particle size of the powders used to prepare the reference and working electrodes usually vary from 2 to 10 mm. In a recent study of the Gibbs energy of formation of MgAl 2 O 4 , [1] the average size of the aluminate particles used, as determined by scanning electron microscopy (SEM), was 3 mm. In the present study, the measurements were made using nanocrystalline MgAl 2 O 4 powders, keeping all other conditions the same.To obtain constant and reversible emf at any given temperature, it was necessary to remove moisture completely in the ceramic enclosure surrounding the cell. Formation of CaO on the surface of CaF 2 , by the reaction of water vapor wi...

Coefficients are tabled on basis of diffusion equation consisting of intrinsic diffusion coefficient D! and extrinsic diffusion coefficient D2: D = Do.! exp(QdRT) + DO•2 exp(-Q2/RT).model for this 'anomalous' diffusion behaviour is the extrinsic vacancy model, in which it is proposed that interstitial impurities in the matrix give rise to an extrinsic vacancy concentration in addition to the intrinsic thermal equilibrium vacancy concentration. The excess diffusion in the fJ phase is attributed to this extrinsic vacancy concentration. 12 -I5 EXPERIMENTAL Sintering was followed essentially by isothermal dilatome try. Two types of titanium powder were studied:(i) irregular shaped sponge fines (ii) spherical powder obtained by vacuum centrifugal atomisation. The characteristics of the powders are given in Table 2 and shown in Fig. 1. Compacts were made from the sponge fines by isomechanical compaction to a green density of about 65% theoretical. The atomised powder was not amenable to cold compaction and compacts were obtained by presintering of tap filled moulds at 1023 K for 30 min. The density in this case was about 66% theoretical.A vacuum dilatometer with a platinum wound furnace capable of operating at temperatures up to 1600 K was employed. The dilatometer, which incorporated an impermeable alumina tube, was fabricated in house. Sintering was carried out at a vacuum of 10-3 Pa. The compact was pushed from the cold end of the evacuated furnace tube into the hot zone maintained at the isothermal sintering temperature, and anchored by means of an alumina push rod. At the other end of the tube the shrinkage was monitored as indicated on a sensitive dial gauge (0'001 mm sensitivity, ± 5 mm range) using an alumina transducer.A PtjPt-13Rh thermocouple passing through the transducer tube was used to measure the compact temperature directly.The sensitivity of 0·001 mm was adequate to record the thermal expansion of the system together with that of the specimen. Thermal expansion of a metal specimen 20 mm in length will be of the order of 0·020 mm at t'.' 1250 K, while the simultaneous expansion of the alumina tube would lead to a much lower reading on the dial gauge. To correct for thermal expansion effects, specimens of solid titanium were used in blank runs at the various isothermal hold temperatures. Reproducibility was confirmed over several blank runs to assure that the shrinkage values obtained from the sintering runs were valid. 9 10 '11 130'6 3·58 X 10-4 152·8 251'2 131 1·9 X 10-3 1·09 4·54 x 10-4Table 1 Diffusion coeflicients* for p titaniumSintering of titanium in its high temperature fJ phase was studied by isothermal dilatometry. The sintering shrinkage y did not follow the normal time exponent type of behaviour, instead being described by the equation y = Kt m /[ 1 -(A + Bt)2J, where m = 1·93 ± 0,07, with an activation energy of 62-90 kJ mol-I. A detailed analysis of these results, based on the 'anomalous' diffusion beha viour reported for fJ titanium, is carried out. It is shown that the generation of a h...

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